Photopolymerizable elements of a photopolymerizable layer sandwiched between a support film and a cover film such as disclosed in U.S. Pat. No. 3,469,982 have achieved widespread usage in the process for making a printed circuit as described in that patent. A preliminary step in this process is to remove the cover film from the element and then to laminate the exposed surface of the photopolymerizable layer to the surface, usually of copper, of a substrate. The support film has sufficient adhesion to the layer that removal of the cover film does not cause the layer to detach from the support film. Sometime after the lamination step, either before or after the step of imagewise exposure of the photopolymerizable layer, but prior to the solvent development step, the support film has to be stripped away from the photopolymerizable layer. At this time, it is desired that the adhesion between the layer and support film not be so high that this stripping causes the layer to detach from the substrate surface and/or lessen the possibility of the support sheet tearing.
In a related phenomenon, the force required to peel the support film from the laminated layer may oscillate widely between a maximum and minimum value as the support is removed to give a characteristic audible sound analogous to the functioning of a zipper. When the difference between the maximum and minimum value is large, detachment of the layer from the substrate surface is more likely to occur.
While this delicate balance of adhesion has been admirably achieved, there nevertheless has existed a need to render the balance less delicate, so that the photopolymerizable element has a wide, reliable processing latitude to accommodate the different cleaning (of the substrate surface), lamination, and stripping conditions practiced by the many users of the element without introducing unwanted deleterious effects during etching or plating.
Other photopolymerizable elements having a similar sandwich structure, such as disclosed in U.S. Pat. No. 3,649,268, have achieved widespread usage in the process of making 4 color surprint proofs directly from contact transparencies to be used in preparing printing plates. In this process, a cover film is removed from a tacky photopolymerizable layer which is then laminated to a white cardboard substrate. The laminated element is exposed to actinic radiation through one of the contact transparencies to produce nontacky exposed image areas and the superposed support film is then removed. Suitable particulate toner is applied to the imaged surface to form an accurate positive copy of the original transparency. Using the previously toned image as a substrate and by repeating several times the lamination, exposure in register, and toning steps with suitable transparencies and toners, a multicolored surprint is formed. In this process a problem known as "pick-off" can occur during the removal of the support film from the exposed laminated element when the support film's adhesion to unexposed photopolymer is too high. In this instance, the unexposed area over a previously toned image area directly below is pulled up with some of the underlying toned surface as the support film is removed. While the balance of adhesion and cohesion has been achieved to reduce the pick-off problem, there nevertheless has existed a need to render the balance less delicate, so that the photopolymerizable element has a wide, reliable processing latitude without unduly increasing support adhesion.